Replace global IntegerKeyValueSetting with per-view CatalogZone model

Track the catalog zone SOA serial per view instead of a single global value.
The new CatalogZone model has a 1:1 FK to netbox_dns.models.View and its own
soa_serial field; each view's catalog zone now increments independently. The
existing global serial is preserved during migration by copying it into every
view's new CatalogZone row.

Change CatalogZoneMemberIdentifier.name from globally unique to unique per
catalog zone (UniqueConstraint(name, catalog_zone)) and add a catalog_zone FK
to enforce it.

Remove the dns-settings management command, which was a generic interface over
the now-removed IntegerKeyValueSetting model.

Increment both catalog zone serials when a zone moves between views: the old
view's catalog zone loses a member and the new view's gains one, so both must
be bumped. Also refresh the member identifier's catalog_zone FK on a view move
so it does not go stale. The pre_save handler now captures _old_view_id and the
post_save handler detects view changes, increments the old view's serial inside
on_commit (before the new view's), and re-creates the identifier.

AI generated, code manually verified.

CHANGELOG.md

@@ -1,4 +1,15 @@
 ## Unreleased
+- Replace the global `IntegerKeyValueSetting` model (single `catalog-zone-soa-serial`
+  value shared by all catalog zones) with a new `CatalogZone` model that has a 1:1
+  foreign key to `netbox_dns.models.View` and its own `soa_serial` field. Each view's
+  catalog zone now tracks and increments its SOA serial independently. Existing serials
+  are preserved during migration by copying the old global value into every view's new
+  `CatalogZone` row.
+- Change the `CatalogZoneMemberIdentifier.name` uniqueness from globally unique to
+  unique per catalog zone (`UniqueConstraint(name, catalog_zone)`). A `catalog_zone`
+  foreign key has been added to the identifier model to enforce this.
+- Remove the `dns-settings` management command, which was a generic get/set/list
+  interface over `IntegerKeyValueSetting` (now removed).
 - Defer the catalog zone SOA serial increment on zone deletion to `transaction.on_commit`, matching
   the create/modify path. Previously the increment ran immediately at `post_delete` time, so a
   rollback of the delete transaction would have left the catalog serial bumped anyway.